Among the 1.7 million new cases of breast cancer diagnosed globally each year, 15-20% are triple-negative breast cancer (TNBC), which has an aggressive phenotype with high metastatic capability and poor prognosis. However, current therapeutics for TNBC have unpredictable efficacy, poor biomarker availability, serious side effects, and the development of treatment resistance. The goal of this proposal is to develop our Cargocyte' technology (patents pending) to improve immune-oncology treatment in patients with TNBC by achieving the safe, non-toxic, targeted, and transient delivery of interleukin 12 (IL-12). IL-12 is a potent inflammatory cytokine and immune activator that attracted significant clinical interest because of its ability to induce a durable anti-tumor response in pre-clinical studies. However, initial clinical development was halted because systemic administration of IL-12 was associated with high levels of systemic toxicity, low response rates, and a narrow therapeutic window. Cargocytes possess unique advantages that facilitate delivery of IL-12 to the tumor microenvironment. They are transfected with IL-12 mRNAs that transform Cargocytes into cell-like factories producing high levels of secreted, bioactive, heterodimeric IL-12 cytokine (CA-IL-12). Compared with passive mechanisms of IL-12 delivery (purified IL-12 cytokine or mRNA, nanoparticles, exosomes etc.), Cargocytes deliver CA-IL-12 to the tumor microenvironment using multiple active cellular mechanisms that uniquely transform the tumor microenvironment from immunologically cold to hot. Furthermore, Cargocytes are tumor trophic and adhere to extracellular matrix (ECM) proteins within the tumor environment, which improves tumor retention and local CA-IL-12 delivery following IT administration. The proposed research in phase I will focus on (Aim 1) evaluating the ability of IT CA-IL-12 in combination with PD-1/PD-Ll immune checkpoint inhibition to target cancer progression using a stringent 4Tl murine orthotopic model of metastatic TNBC. Aim 2 will evaluate CA-IL-12/PD-Ll safety and toxicity in preparation for a type C pre-IND meeting using the 4Tl orthotopic model of metastatic TNBC. Following completion of these studies and guidance discussions with FDA regulatory personnel, further IND enabling work will be pursued by a phase II STTR proposal to develop CA-IL-12 for clinical applications.